An electrophotographic image forming apparatus performs image formation by: forming an electrostatic latent image based on image data on the surface of a photoreceptor drum uniformly charged using electrostatic force by an optical write device; developing this electrostatic latent image with toner by a developing device; transferring the resulting toner image to a recording sheet; and then fusing and fixing the toner image to the recording sheet by a fixing device applying heat and pressure to the toner image.
Transfer devices of the charger type have been widely used for image forming apparatus because their structures are simple. In recent years, however, transfer devices of the contact type have become mainstream because the charger type transfer devices generate ozone during discharge for obtaining a transfer power output, thus giving off an unpleasant odor and raising a problem of health.
Such contact type transfer devices include: those which are configured to bring a transfer electrode such as an electrically conductive roller or brush into direct contact with the reverse side of a recording sheet to cause a toner image formed on an image carrier to be transferred to the recording sheet; and those which are configured to interpose a carrier member such as an electrically conductive endless belt or film between a photoreceptor drum and a transfer electrode to achieve image transfer. A transfer roller and a transfer belt used as the transfer electrode and the carrier member, respectively, in these contact type transfer devices need to have predetermined elasticity and pressure in order to stabilize the recording sheet nipping condition of the photoreceptor drum and transfer roller and, therefore, their respective surface portions are formed of an elastic conductive material having a high resistance.
Such high-resistant, elastic conductive materials to be used for the surface portion of such a transfer roller and for such a transfer member as a transfer belt are generally known to have a voltage or current dependence of resistance. Conventional contact type transfer devices use an elastic conductive material having a suppressed voltage or current dependence of resistance for the transfer member in order to stabilize the output of transfer power which influences the transfer process. Among such conventional transfer devices there is known one which has a transfer roller having an absolute value of voltage dependence of resistance ? log R/?V{(log O/kV)} set to not more than 0.5 (see Japanese Patent Laid-Open Publication No. H10-133496 for example). The purpose of this art is to prevent the occurrence of non-uniformity in the density of an image due to variations in transfer efficiency by making uniform the transfer voltage acting on a recording sheet and toner during the transfer process.
If entrance- and exit-side paper guides with respect to the contact position between the surface of a photoreceptor and a transfer member, as well as a fixing device and the like are charged up in an image forming apparatus, it is likely that paper jam, electrostatic discharge damage to an electric circuit or the like due to a high voltage, abnormal discharge, and the like occur. In such an image forming apparatus the transfer member is grounded through a resistance of several hundred KO as measures to avoid such inconveniences.
Further, in order to enhance the release property of a recording sheet from the surface of the transfer member and prevent toner from scattering during the transfer process, the transfer member is grounded or applied with a relatively low voltage. For this reason, there are formed a primary transfer current path passing through the transfer member, recording sheet and photoreceptor from the power source and, in addition, a secondary transfer current path passing through the transfer member from the power source in a different direction than the primary transfer current path. Thus, current passes on the secondary transfer current path also.
If transfer current fluctuations occur in the primary transfer current path including the transfer member and recording sheet due to partial resistance fluctuations in the primary transfer current path, a voltage drop by the transfer member fluctuates to cause the intensity of the transfer electric field in the transfer region to vary. The above-described arrangement can suppress such a variation in the intensity of the transfer electric field to ensure a stabilized transfer operation.
However, as the transfer efficiency of the transfer process, which determines the condition of image formation on a recording sheet, is susceptible to changes with time of the components of the device and to environmental conditions including temperature and humidity, if the voltage- or current-dependence of resistance of the transfer member is suppressed, need for strict control over other electrical conditions arises to ensure a satisfactory condition of image formation on the contrary, thus resulting in a problem that control of power supply to the transfer device during image formation becomes complicated.
An object of the present invention is to provide a belt transfer apparatus which is capable of reducing the susceptibility of the transfer efficiency of the transfer process to changes with time of the components of the device and changes of environmental conditions while facilitating control over power supply to the transfer device during image formation by imparting an appropriate voltage- or current-dependence to the resistance inherent to an elastic conductive material forming a transfer belt serving as the transfer member or to each of resistances of the elastic conductive material in different directions, as well as an image forming apparatus provided with such a belt transfer apparatus.